Methods for increasing a retention force between a polymeric scaffold and a delivery balloon

ABSTRACT

A medical device-includes a scaffold crimped to a catheter having an expansion balloon. The scaffold is crimped to the balloon by a process that includes inflating the delivery balloon during a diameter reduction to improve scaffold retention and maintaining an inflated balloon during the diameter reduction and prior and subsequent dwell periods.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to drug-eluting medical devices; moreparticularly, this invention relates to processes for crimping apolymeric scaffold to a delivery balloon.

2. Background of the Invention

The art recognizes a variety of factors that affect a polymericscaffold's ability to retain its structural integrity when subjected toexternal loadings, such as crimping and balloon expansion forces. Theseinteractions are complex and the mechanisms of action not fullyunderstood. According to the art, characteristics differentiating apolymeric, bio-absorbable scaffolding of the type expanded to a deployedstate by plastic deformation from a similarly functioning metal stentare many and significant. Indeed, several of the accepted analytic orempirical methods/models used to predict the behavior of metallic stentstend to be unreliable, if not inappropriate, as methods/models forreliably and consistently predicting the highly non-linear behavior of apolymeric load-bearing portion of a balloon-expandable scaffold(hereinafter “scaffold”). The models are not generally capable ofproviding an acceptable degree of certainty required for purposes ofimplanting the scaffold within a body, or predicting/anticipating theempirical data.

Moreover, it is recognized that the state of the art in medicaldevice-related balloon fabrication, e.g., non-compliant balloons forscaffold deployment and/or angioplasty, provide only limited informationabout how a polymeric material might behave when used to support a lumenwithin a living being via plastic deformation of a network of ringsinterconnected by struts. In short, methods devised to improvemechanical features of an inflated, thin-walled balloon structure, mostanalogous to mechanical properties of a pre-loaded membrane when theballoon is inflated and supporting a lumen, simply provides little, ifany insight into the behavior of a deployed scaffold. One difference,for example, is the propensity for fracture or cracks to develop in ascaffold. The art recognizes the mechanical problem as too different toprovide helpful insights, therefore, despite a shared similarity inclass of material. At best, the balloon fabrication art provides onlygeneral guidance for one seeking to improve characteristics of ascaffold.

Polymer material considered for use as a scaffold, e.g. PLLA or PLGA,may be described, through comparison with a metallic material used toform a scaffold, in some of the following ways. A suitable polymer has alow strength to weight ratio, which means more material is needed toprovide an equivalent mechanical property to that of a metal. Therefore,struts must be made thicker and wider to have the strength needed. Thescaffolding also tends to be brittle or have limited fracture toughness.The anisotropic and rate-dependant inelastic properties (i.e.,strength/stiffness of the material varies depending upon the rate atwhich the material is deformed) inherent in the material only compoundthis complexity in working with a polymer, particularly, bio-absorbablepolymer such as PLLA or PLGA.

Processing steps performed on, design changes made to a metal stent thathave not typically raised concerns for, or require careful attention tounanticipated changes in the average mechanical properties of thematerial, therefore, may not also apply to a scaffold due to thenon-linear and sometimes unpredictable nature of the mechanicalproperties of the polymer under a similar loading condition. It issometimes the case that one needs to undertake extensive validationbefore it even becomes possible to predict more generally whether aparticular condition is due to one factor or another—e.g., was a defectthe result of one or more steps of a fabrication process, or one or moresteps in a process that takes place after scaffold fabrication, e.g.,crimping. As a consequence, a change to a fabrication process,post-fabrication process or even relatively minor changes to a scaffoldpattern design must, generally speaking, be investigated more thoroughlythan if a metallic material were used instead of the polymer. Itfollows, therefore, that when choosing among different scaffold designsfor improvement thereof, there are far less inferences, theories, orsystematic methods of discovery available, as a tool for steering oneclear of unproductive paths, and towards more productive paths forimprovement, than when making changes in a metal stent.

It is recognized, therefore, that, whereas inferences previouslyaccepted in the art for stent validation or feasibility when anisotropic and ductile metallic material was used, such inferences wouldbe inappropriate for a scaffold. A change in a scaffold pattern mayaffect, not only the stiffness or lumen coverage of the scaffold in itsdeployed state supporting a lumen, but also the propensity for fracturesto develop when the scaffold is crimped or being deployed. This meansthat, in comparison to a metallic stent, there is generally noassumption that can be made as to whether a changed scaffold pattern maynot produce an adverse outcome, or require a significant change in aprocessing step (e.g., tube forming, laser cutting, crimping, etc.).Simply put, the highly favorable, inherent properties of a metal(generally invariant stress/strain properties with respect to the rateof deformation or the direction of loading, and the material's ductilenature), which simplify the stent fabrication process, allow forinferences to be more easily drawn between a changed stent patternand/or a processing step and the ability for the stent to be reliablymanufactured with the new pattern and without defects when implantedwithin a living being.

A change in the pattern of the struts and rings of a scaffold that isplastically deformed, both when crimped to, and when later deployed by aballoon, unfortunately, is not as easy to predict as a metal stent.Indeed, it is recognized that unexpected problems may arise in scaffoldfabrication steps as a result of a changed pattern that would not havenecessitated any changes if the pattern was instead formed from a metaltube. In contrast to changes in a metallic stent pattern, a change in ascaffold pattern may necessitate other modifications in fabricationsteps or post-fabrication processing, such as crimping andsterilization.

One problem frequently encountered with a scaffold for delivery to asite in a body using a balloon is reliably retaining the scaffold on theballoon as it passes through tortuous anatomy. If the scaffold is notheld on the balloon with sufficient force, it can slip off of theballoon during transit to the target site. For a metallic stent, thereare several approaches proposed for increasing the retention of thestent to a balloon during transit to the target site. However, methodsproposed thus far for retaining the scaffold on a balloon are in need ofimprovement.

In one example of a method for crimping a metallic stent to a deliveryballoon, the stent is placed in a crimper and the temperature elevatedto facilitate greater compliance in the balloon material to allowmaterial to extend between gaps in the stent struts. Additionally,balloon pressure is maintained while the stent is being crimped toincrease stent retention to the balloon. After an initial pre-crimp, thestent is placed on the delivery balloon and allowed to slightly recoilunder balloon pressure and while the stent has an elevated temperature.After this step, the stent is crimped onto the balloon while the balloonis pressurized. The stent is cycled to larger and smaller diameters.Additionally, balloon pressure may be supplied in bursts or heldconstant during these crimping steps. Further details of this processmay be found in U.S. application Ser. No. 12/895,646 filed Sep. 30,2010. This application shares a common inventor with the presentapplication.

In light of the foregoing problems, there is a need for improving theretention of a scaffold on a balloon while avoiding adverse effects onthe mechanical characteristics of the scaffold when the scaffold isfully deployed to support a lumen.

SUMMARY OF THE INVENTION

It has been found that the retention force of a crimped polymer scaffoldon delivery balloon may be increased by a process that includes crimpingthe scaffold to the balloon while the balloon is pressurized; that is,the balloon is pressurized at the same time as the scaffold's diameteris being reduced by crimper blades. Additional features of the crimpingprocess include heating the scaffold to a temperature close to, butbelow the glass transition temperature (TG) of the polymer material andapplying balloon pressure during dwell periods (i.e., balloon pressureis applied when the scaffold diameter is held constant).

The process includes several crimping steps. Following each crimpingstep, a dwell period occurs in which balloon pressure is supplied tocorrect any misalignments in scaffold struts, and/or deter furthermisalignment in subsequent crimping steps. After one or more initialcrimping steps, the partially-crimped scaffold is removed from thecrimper head to check alignment on the balloon. This step is referred toas the final alignment, or check final alignment step in the disclosure.After checking alignment, the scaffold is returned to the crimper toperform the final crimp. The final diameter reduction is performed whilethe balloon is pressurized to urge balloon material between gaps in thescaffold struts. It was found that there was a significant increase inthe retention or pull-off force of the crimped scaffold on the balloonwhen the balloon was pressurized during the final crimping step.

In one aspect of the invention a scaffold crimped while balloon pressureis applied produces at least a 0.5 lb increase in retention force over ascaffold crimped using the same process but without balloon pressuresupplied during the final crimp step. The scaffold may be manufacturedfrom a precursor expanded into a tube then cut into a scaffold. Thematerial may be a polymer comprising PLLA or PLGA. The crimpingtemperature may be between 5 and 15 degrees below a lower value for thepolymer's TG.

In another aspect of the invention, a scaffold is reduced in diameter bya factor of at least 2.5 when crimped and has a retention force of above1.0 lb. The crimping process includes pressurizing the balloon after thescaffold diameter is reduced by about 50% from its pre-crimp diameter.

In another aspect of the invention, the balloon pressure during dwellperiods prior to a final alignment is about twice the balloon pressureapplied after final alignment.

According to another aspect of the disclosure, a first crimping amountoccurs before removing the scaffold to check alignment and a secondcrimping amount occurs after alignment. Greater balloon material can beurged between scaffold struts when the scaffold has a larger diameter;however, when at larger diameters the scaffold tends to move about moreover the balloon (it also becomes more difficult to make longitudinaladjustments to scaffold position with respect to balloon markers ifballoon material is urged between scaffold struts at the time of thefirst crimping amount). According to this aspect of the invention, thefirst and second amount of crimping are selected, in part, to crimp thescaffold sufficiently to enable adjustment of scaffold alignment on theballoon to achieve final alignment so that it will not shift on theballoon while leaving sufficiently-sized gaps between struts so thatballoon material may be urged into gaps during the final crimp toincrease scaffold retention force. In one embodiment the scaffold isreduced in diameter by about 50%, and then removed to check alignment.Then the scaffold is crimped by an additional about 33% to arrive at afinal crimped diameter. The tube forming the scaffold has a diameter atabout a deployed size or greater than a deployed size in thisembodiment. For scaffold formed from a tube having a diameter less thanor greater than a deployed size, each having the same final crimp size,the about 50% reduction may be correspondingly less than 50% or greaterthan 50%, respectively.

According to one embodiment, the ratio of diameter reduction beforefinal alignment to the diameter reduction after final alignment may beabout 1.5. For example, the scaffold diameter may be reduced by 50%before final alignment, and then reduced by 33% after final alignment.

The amount of diameter reduction for a single crimping step may belimited to no more than a 40% reduction in diameter, to allow for stressrelaxation in the polymer material. The amount of diameter reduction andrate optimal for a scaffold will generally depend on the material andproximity to TG of the scaffold temperature during crimping. Forexample, the diameter reductions for a process that has three crimpingsteps may be about 20%, 40% and 30% for a PLLA scaffold and crimpingtemperature of about 48 Deg Celsius. The scaffold diameter may beinitially reduced in size by 20%, followed by a dwell period, then thediameter is reduced in size by 38%, followed by a second dwell period,and then reduced to its final crimped diameter by reducing the diameterby an additional 33% where balloon pressure is supplied to increasescaffold retention during the final step. For these crimping percentagesa 3.5 mm scaffold would be reduced in diameter size from 3.5 mm to 1.19mm.

According to another aspect of the invention, a method for crimping aballoon-expanded scaffold to a balloon includes the steps of providing atube comprising PLLA characterized by a glass transition temperaturerange having a lower limit of about TG-low; radially expanding the tubeto increase its radial stiffness; forming the scaffold from theradially-expanded tube, including the steps of forming a circumferentialseries of closed cells having a W-shape and linear link strutsconnecting the W-shape cells; and crimping the scaffold to the balloonwhile the scaffold has a crimping temperature of between about 5 to 15degrees below TG-low including maintaining a pressure in the balloon asthe scaffold diameter is being reduced in size.

According to another aspect of the invention, a method for securing ascaffold to a balloon includes the steps of crimping the scaffold to theballoon while the scaffold has a temperature of between about TG and 15degrees below TG, the scaffold being made from a polymer comprising PLLAor PLGA, the crimping step including the steps of (a) reducing thescaffold diameter from a starting size to a first size, (b) maintainingthe first size for a first dwell period to allow for stress relaxationin the polymer, (c) reducing the scaffold diameter from the first sizeto a second size, (d) maintaining the second size for a second dwellperiod to allow for stress relaxation in the polymer while the balloonhas a first pressure, (e) reducing the scaffold diameter from the secondsize to a third size, (f) maintaining the third size for a third dwellperiod to allow for stress relaxation in the polymer while the balloonhas the first pressure, and (g) reducing the scaffold diameter from thethird size to a fourth size, wherein the balloon has a second pressurewhile the scaffold diameter is being reduced from the third size to thefourth size.

The scope of the methods and apparatus of the invention also encompassprocesses that may be used to crimp a scaffold as substantiallydescribed in US Pub. No. 2010/0004735 and US Pub. No. 2008/0275537. Thethickness of the tube from which the scaffold is formed may have athickness of between 0.10 mm and 0.18 mm, and more narrowly at or about0.152 mm. The scaffold may be made from PLLA. And the scaffold may becrimped to a PEBAX balloon.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference, and as if eachsaid individual publication or patent application was fully set forth,including any figures, herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a flow diagram for fabricating a scaffold and crimping thefabricated scaffold to a balloon according to the disclosure.

FIG. 1B shows the crimping process portion of FIG. 1A in graphical form,plotting scaffold diameter vs. time and indicating the balloon pressuresupplied during steps of the crimping process.

FIG. 2 is a planar view of a portion of a scaffold that was crimped to aballoon according to aspects of the disclosure. This view describes thescaffold pattern of the load-bearing structure of the scaffold that iscrimped to a balloon according to the process of FIG. 1A-1B.

DETAILED DESCRIPTION OF EMBODIMENTS

The “glass transition temperature,” TG, is the temperature at which theamorphous domains of a polymer generally change from a brittle, vitreousstate to a solid deformable or ductile state at atmospheric pressure. Inother words, the TG corresponds to the temperature where the onset ofnoticeable segmental motion in the chains of the polymer occurs. When anamorphous or semi-crystalline polymer is exposed to an increasingtemperature, the coefficient of expansion and the heat capacity of thepolymer both increase as the temperature is raised, indicating increasedmolecular motion. As the temperature is raised the actual molecularvolume in the sample remains constant, and so a higher coefficient ofexpansion points to an increase in free volume associated with thesystem and therefore increased freedom for the molecules to move. Theincreasing heat capacity corresponds to an increase in heat dissipationthrough movement. TG of a given polymer can be dependent on the heatingrate and can be influenced by the thermal history of the polymer.Furthermore, the chemical structure of the polymer heavily influencesthe glass transition by affecting mobility.

Poly(lactide-co-glycolide) (PLGA) and Poly(L-lactide) (PLLA) areexamples of a class of semi-crystalline polymers that may be used toform the scaffolds described herein. PLLA is a homopolymer and PLGA is aco-polymer. The percentage of glycolide (GA) in a scaffold constructedof PLGA may vary, which can influence the lower range of TG. Forexample, the percentage of GA in the matrix material may vary between0-15%. For PLLA, the onset of glass transition occurs at about 55degrees Celsius. With an increase of GA from about 0% to 15% the lowerrange for TG for PLGA can be correspondingly lower by about 5 degreesCelsius.

In one embodiment, a tube is formed by an extrusion of PLLA. The tubeforming process described in US Pub. No. 2010/00025894 may be used toform this tube. The finished, solidified polymeric tube of PLLA may thenbe deformed in radial and axial directions by a blow molding processwherein deformation occurs progressively at a predetermined longitudinalspeed along the longitudinal axis of the tube. For example, blow moldingcan be performed as described in U.S. Publication No. 2009/0001633. Thisbiaxial deformation, after the tube is formed, can produce noticeableimprovement in the mechanical properties of the scaffold structuralmembers cut from the tube without this expansion. The degree of radialexpansion that the polymer tube undergoes characterizes the degree ofinduced circumferential molecular or crystal orientation. In a preferredembodiment, the radial expansion ratio or RE ratio is about 450% of thestarting tube's inner diameter and the axial expansion ratio or AE ratiois about 150% of the starting tube's length. The ratios RA and AE aredefined in US Pub. No. 2010/00025894.

The above scaffold's outer diameter may be designated by where it isexpected to be used, e.g., a specific location or area in the body. Theouter diameter, however, is usually only an approximation of what willbe needed during the procedure. For instance, there may be extensivecalcification that breaks down once a therapeutic agent takes effect,which can cause the scaffold to dislodge in the vessel. Further, since avessel wall cannot be assumed as circular in cross-section, and itsactual size only an approximation, a physician can choose to over-extendthe scaffold to ensure it stays in place. For this reason, it ispreferred to use a tube with a diameter larger than the expecteddeployed diameter of the scaffold.

In one embodiment the ratio of deployed to fully crimped diameter isabout 2.5. In this embodiment, the crimped diameter corresponds to anouter diameter that is only about 40% of the starting diameter. Hence,when deployed the drug eluting scaffold is expected to increase in sizeat least to about 2.5 times its crimped diameter size.

In one particular example, a scaffold is formed from a biaxiallyexpanded tube having an outer diameter of 3.5 mm, which approximatelycorresponds to a deployed diameter (the scaffold may be safely expandedup to 4.0 mm within a lumen). The iris of the crimping mechanism reachesa diameter of 0.044 in, which is maintained for a 200 sec dwell period(i.e., scaffold held at 0.044 in outer diameter within crimpingmechanism). When later removed from the crimper, the scaffold willrecoil despite there being a restraining sheath placed over the scaffoldimmediately after the scaffold is removed from eth crimper. The scaffoldand sheath are then subjected to radiation sterilization. At the pointof use, i.e., at the point in time when a medical specialist removes therestraining sheath, the scaffold has an outer diameter of about 0.052 in(1.32 mm), or about 37% of the starting tube diameter of 3.5 mm. When inthe crimping mechanism the scaffold reaches about 34% of the startingtube size.

As discussed earlier, fabrication of a scaffold presents challenges thatare not present in metallic stents. One challenge, in particular, is thefabrication of a scaffold, which means the load bearing network ofstruts including connectors linking ring elements or members thatprovide the radial strength and stiffness needed to support a lumen. Inparticular, there exists an ongoing challenge in fabricating a scaffoldthat is capable of undergoing a significant degree of plasticdeformation without loss of strength, e.g., cracks or fracture ofstruts. In the disclosed embodiments, a scaffold is capable of beingdeformed from a crimped diameter to at least 2.5 times the crimpeddiameter without significant loss of strength.

A problem encountered with fabrication of a scaffold for delivery to asite in a body using a balloon is the ability of the scaffold to besafely crimped to the balloon so that an adequate retention force isestablished between the scaffold and balloon. A “retention force” for ascaffold crimped to a balloon means the maximum force, applied to thescaffold along the direction of travel through a vessel that thescaffold-balloon is able to resist before dislodging the scaffold fromthe balloon. The retention force for a scaffold on a balloon is set by acrimping process, whereby the scaffold is plastically deformed onto theballoon surface to form a fit that resists dislodgment of the scaffoldfrom the balloon. Factors affecting the retention of a scaffold on aballoon are many. They include the extent of surface-to-surface contactbetween the balloon and scaffold, the coefficient of friction of theballoon and scaffold surfaces, and the degree of protrusion or extensionof balloon material between struts of the scaffold. As such, the pulloff or retention force for a scaffold generally varies with its length.Therefore the shorter the scaffold the more likely it can becomedislodged when the catheter is pushed through tortuous anatomy.

For a metal stent there are a wide variety of methods known forimproving the retention force of a stent on a balloon via modificationof one or more of the foregoing properties; however, many are notsuitable or of limited usefulness for a scaffold, due to differences inmechanical characteristics of a scaffold verses a metal stent asdiscussed earlier. Most notable among these differences is brittlenessof polymer material suitable for balloon-expanded scaffold fabrication,verses that of a metal stent, and the sensitivity of the polymermaterial to heat. Whereas a metal stent may be deformed sufficiently toobtain a desired retention force, the range of deformation available toa polymer scaffold, while avoiding cracking or fracture-relatedproblems, by comparison, is quite limited. The application of heat hasbeen shown as effective for increasing retention forces for metalstents. However, the heat levels used can cause detrimental effects tothe polymer material since they tend to correspond to temperatures wellwithin, or above the TG of the material. For this reason, known heatmethods for increasing retention forces for metal stents tend to beviewed as inappropriate for increasing a retention force between ascaffold and balloon.

The invention addresses the unique challenges presented by a scaffoldthat needs to be retained on a balloon. These challenges are present forseveral reasons. First, there is less space available between struts ina crimped state, which prevents balloon material from extending betweenstruts. As a result, there is less abutment or interference betweenstruts and balloon material, which interference/abutment has previouslybeen relied upon to increase the retention force of a metal stent on aballoon. This condition is a result of the need to fabricate wider andthicker struts for the scaffold, as compared to a metal stent, so as toprovide adequate, deployed radial strength. Second, as indicated above,a polymer is more sensitive to temperature ranges that have previouslybeen used to increase retention to a balloon. Heating of a scaffold towithin, or above TG induces significant changes in the molecularorientation of the polymer material that result in loss of strength whenthe scaffold is plastically deformed to its deployed diameter.

The art has previously devised methods for retaining a scaffold on adelivery balloon in response to these challenges. In one example, thescaffold is crimped to the delivery balloon at a temperature well belowthe polymer's TG. Then the scaffold, disposed between ends of theballoon, is thermally insulated from the balloon's ends. The ends of theballoon are then heated to about 185 degrees Fahrenheit to expand thediameter of the balloon material at its ends. The expanded balloon endsform raised edges abutting the scaffold ends to resist dislodgment ofthe scaffold from the balloon. In one example, this process provided aretention force of about 0.35 lb. for a Poly(L-lactide) (PLLA) scaffoldcrimped to a polymide-polyether block co-polymer (PEBAX) balloon. Anexample of this process is disclosed in U.S. Pat. No. 6,666,880.

U.S. patent application Ser. No. 12/772,116 filed Apr. 30, 2010 (“'116application) discusses a study that was conducted to investigate theeffects on retention forces for crimped scaffolds. Principally, thisstudy identified a temperature range relative to a TG of the scaffoldmaterial that improved retention forces without detrimentally affectingscaffold mechanical properties when deployed to support a vessel. ForPLLA it was found that modifying the pressure and hold time of thescaffold for crimping temperatures of between 40° and 55° C. improvedthe scaffold retention, with about 45-51° C. and 48° C. being preferredtemperatures for a PLLA scaffold. Additionally, the '116 applicationfound that retention forces could be improved if the scaffold werecrimped down to an intermediate diameter and then the balloon isdeflated then re-inflated, followed by crimping the scaffold down to afinal crimp diameter. The '116 application also contemplates similarresults for PLGA, if TG for this material is taken into considerationand assuming other characteristics of the process and scaffold pattern.For PLGA having % GA of about 5% the temperature ranges for crimping maybe between about 46 to 53 degrees Celsius. For PLGA having % GA of about15% the temperature ranges for crimping are about 43 to 50 degreesCelsius.

When the scaffold is crimped to a balloon while being heated totemperatures well within the range of TG for the scaffold polymer, thereis a greater tendency for polymer chain re-alignment to occur that willresult in loss of strength when the scaffold is later deployed.Unacceptable crack formation (either in the number or extent of cracks),voids or outright fracture was observed in subsequent testing. If thecrimping temperature is raised too high relative to the TG of thepolymer, the memory of the matrix material at the starting tubingdiameter is being removed, or reformed as the scaffold is deformed. As aconsequence, when the scaffold is later expanded under physiologicalconditions, e.g., body temperature; it becomes more susceptible to crackformation due to its brittle properties at body temperatures and lack ofchain alignment from its starting diameter. Retention force and scaffoldintegrity when crimped to the balloon generally improves at highertemperatures, however, the scaffold loses its structural integrity whenlater deployed if the temperature is raised too high, e.g., above TG. Onthe other hand, when the scaffold is heated to temperatures below about15 degrees Celsius of the glass transition temperature, or not heated atall, there is no noticeable improvement in scaffold retention. It wasfound that the most effective range was between about 15 degrees belowand up to about TG.

The '116 application explains that the above and related unexpectedresults may be explained in the following manner. When a polymerscaffold is crimped at a temperature slightly below its TG (e.g., from 5to 15 degrees Celsius below TG), there are very short chains of thematrix material that are able to freely move to assist in thedeformation of the scaffold without exceeding material stress limits. Atthe same time, the longer chains of the matrix substantially maintaintheir alignment, and, thus, stay intact without losing their orientationset when the starting tube was expanded. By doing so, the scaffold maybe crimped down to a diameter for good scaffold retention, while theorientation of a majority of polymer chains would be the same to ensuredesirable strength and fracture toughness in the final product, i.e.,when the scaffold is deployed to support a vessel.

FIG. 1 of the '116 application shows a flow for a crimping process for a3.0 mm (0.118 in) scaffold that is crimped to a final crimp diameter of0.044 in. The diameter reduction from 0.118 in to 0.044 in includesthree intermediate crimping diameters of 0.083 in, 0.063 in and 0.07 in,following a “pre-crimp” procedure in which the PLLA scaffold temperatureis raised to a temperature of about 48° C. When the scaffold hasattained the intermediate crimp diameters, the crimper jaws are held atthe crimping diameter for a dwell period of 30 sec, 15 sec and 10 sec,respectively. After the final crimp diameter has been obtained, thecrimp jaws are held at the final crimp diameter for about 200 sec. Thedelivery balloon, i.e., a PEBAX balloon, is inflated to a pressure of 17psi for the dwell period 30, 15 and 10 second dwell periods. The dwellperiods for the intermediate crimping stages are included in the processto allow for stress relaxation in the polymer material before decreasingthe scaffold diameter further. Before the crimper iris is reduced byactuation of the crimper jaws, the balloon is deflated. Thus, in apreferred embodiment of the '116 application whenever the scaffolddiameter is decreased, the balloon is not inflated.

Notwithstanding improved results in stent retention when practicinginventions described in the '116 application, it is desirable to furtherincrease a scaffold retention force. For example, for a coronaryscaffold it is desirable to have a balloon-scaffold retention force(i.e., force required to pull scaffold off balloon) of at least 0.7 lbsand preferably over 1.0 lbs. According to the disclosure, methods of theinvention are expected to increase the retention force on an 18 mmlength, 3.5 mm pre-crimp diameter scaffold by at least 0.5 lbs over theprocess used to produce the data in the '116 application.

Referring to FIGS. 1A-1B there is a flow process and graph,respectively, of a crimping method for a 3.5 mm diameter and 18 mmlength scaffold. The method is discussed herein as a series of five“stages” with diameter reduction steps between stages. Each “stage”refers to a period where the crimper jaws are maintained at a constantdiameter for a dwell period. The scaffold diameter is constant duringthese periods.

For the stages preceding the “final alignment” or “verify finalalignment” step in FIGS. 1A-1B, where the scaffold and balloon areremoved from the crimper to check alignment, the balloon is inflated tominimize further out of plane, or irregular movement or twisting ofstruts initiated in preceding crimping steps during subsequent crimpingsteps. Some of the advantages of inflating a balloon during these dwellperiods to achieve this result are explained in U.S. application Ser.No. 12/861,719 filed Aug. 23, 2010.

As mentioned earlier, a polymer scaffold, and in particular a misalignedpolymer scaffold is more susceptible to damage within a crimper than acorresponding metal stent. A polymer scaffold that has even a “slight”misalignment within the crimper has a far greater chance of becomingdamaged than a metal stent. Of course, the need to avoid twisting orbending in struts of metal stents when in a crimper is known. However,unlike metal stents, which are far more tolerant of local irregular ornon-uniform forces acting on struts through blade edges, polymer strutsare more easily distorted when the crimping forces are non-uniformlyapplied. Due to the proximity of struts to each other (as required sincethicker and wider struts are needed to provide equivalent stiffness to ametal stent and there is sometimes a greater diameter reduction neededduring crimping), there is a greater chance of abutting struts whichleads to out of plane twisting and overlapping scaffold structure in thecrimped state. The effects of irregular or non-uniform crimping forceson a polymer scaffold are therefore more severe than in the case of ametal stent. The differences are most clearly evident in the instancesof cracking and/or fracture in deployed polymer scaffolds that showirregular twisting or bending.

More local support for individual struts during the dwell periods isbelieved to correct for struts predisposed to twist or overlap withadjacent struts (a strut predisposed to twist or overlap with otherstruts refers to a strut that was previously slightly bent or twistedout of plane when the scaffold was at a larger diameter). In essence,balloon pressure during the dwell periods is believed to apply abeneficial correcting force on the luminal side of struts, which canserve to limit a strut's potential to overlap or twist further ascrimper blades are applied in subsequent steps.

When crimped down from a larger diameter (e.g., from 0.136 to 0.11 inFIG. 1A), there is little stabilizing support available for the scaffoldsince its diameter is much larger than the deflated balloon upon whichthe scaffold sits. As such, any initial non-uniform applied crimpingforce, or misalignment, e.g., due to a residual static charge on thepolymer surface, can initiate irregular bending that becomes morepronounced when the scaffold diameter is reduced further. Frictionbetween the blades and the scaffold surface, or residual static chargeor static charge buildup induced by sliding polymer surfaces are alsosuspect causes of this irregular deformation of the scaffold. When theballoon is inflated to support the scaffold from the interior duringdwell periods, the irregular bending and twisting of struts seen at thefinal crimp diameter (when the scaffold is removed from the crimper)were reduced substantially. The scaffold was more able to maintain aproper orientation with respective to the crimper axis.

Referring again to FIG. 1, the scaffold is partially crimped, thenremoved from the crimper to check its alignment on the balloon (StagesI, II, III as the dwell periods). The scaffold is then returned to thecrimper to perform final crimp steps, e.g., Stage IV, reduce to 0.044in, then dwell Stage V. During these final steps the balloon isapproximately at a constant pressure. Unlike the earlier crimping steps,the balloon is pressurized when the scaffold is crimped to the finaldiameter. The presence of balloon pressure during the final crimp (the“intermediate pressure” step), as compared to the same process withoutthe “intermediate pressure” step, i.e., about atmospheric balloonpressure for the final crimp, greatly increased the retention force ofthe scaffold to the balloon. Stated differently, the retention force ofscaffold to balloon was much higher when the balloon is pressurizedduring the final crimp, or diameter reduction step.

It is believed that the greatly increased retention force was achievedbecause the balloon material opposing gaps in scaffold struts during thefinal crimp tended to extend in-between gaps more often as the scaffoldwas crimped due to the opposing balloon pressure applied to the balloonmaterial. Without this pressure, the balloon material tended to deflectaway from the gaps as the size of the gaps narrowed during the finalcrimp. Essentially, the balloon pressure forced more balloon materialinto gaps—rather than deflect the material away from the gaps—when thediameter is being reduced in size.

It should be noted that there was concern over whether the “intermediatepressure” step would cause balloon damage for balloon pressure appliedin an amount that would make a difference in scaffold retention. Thegaps between scaffold struts for a scaffold having significant diameterreductions and relatively thick struts are narrower than for struts of ametal stent. Forcing balloon material into narrower spaces gave rise toconcerns that balloon material would be excessively pinched betweenstruts, thereby causing damage to the balloon. In a preferred embodimentthe pressure applied during the earlier dwell periods (Stages I, II andIII) is about twice that applied during the final crimp steps (150 psiand 70 psi, respectively), as shown in FIGS. 1A-1B. This ratio ofballoon pressure was found to produce good results, despite the previousconcerns. It is contemplated that other pressure ratios, or increasedpressure values may improve results. To the inventors' surprise,however, a relatively modest amount of pressure applied during theintermediate pressure step can produce significant improvement inscaffold retention, so that the risk of damaging a balloon and/orscaffold is reduced.

In other embodiments balloon pressure may be applied in bursts, ratherthan set at a constant level, during the intermediate pressure step.Further, there may be benefits to using balloon pressure during theprior partial crimp steps. Since the scaffold is re-aligned followingthe earlier crimp steps, the benefits of using balloon pressure duringearlier crimping steps are not so much believed to lay in increasedretention. Rather, balloon pressure may help avoid irregular twisting orbending in scaffold struts (for the reasons discussed above) as thescaffold diameter is being reduced.

EXAMPLES

Further details of the FIG. 1A flow process for a 3.5 mm scaffoldmanufacture and crimping to a delivery balloon will now be discussed.FIG. 1B illustrates in graphical form the crimping portion of the FIG.1A flow—a graph of scaffold diameter verses time with a balloon pressureof 150 psi or 70 psi applied during the dwell periods and theintermediate pressure step (i.e., crimping between Stage IV and StageV). The scaffold was crimped using a crimper having film-sheets disposedbetween the metal crimper blades and the scaffold. This particular typeof crimper is discussed in greater detail below.

As discussed above, the scaffold is formed from a PLLA or PLGAprecursor, including a biaxial expansion of the precursor to form atube, followed by laser cutting the scaffold from the tube. Next, apre-crimp procedure is performed, which includes placing the scaffoldbetween the balloon markers and aligning the scaffold with the iris ofthe crimper. Using an anti-static air gun, both the scaffold andinterior of the iris chamber are deionized. The deionization step wasfound necessary to reduce misalignments of the scaffold resulting from astatic charge buildup caused by sliding contact between polymersurfaces, as explained in more detail in U.S. application Ser. No.12/776,317 filed May 7, 2010 (62571.398).

Stage I:

The scaffold (supported on the balloon of the balloon-catheter) isplaced within the crimp head. The crimping temperature is obtained byheating the crimper jaws to an appropriate temperature and then bringingthe jaws into thermal contact with the scaffold. The crimper jaws areset to 0.136 in and maintained in this position for about 10 seconds toallow the scaffold temperature to increase to a crimping temperaturethat is near to, but below the TG of the scaffold material (e.g., thecrimping temperature for a PLLA scaffold of FIG. 1 is 48+/−3° C.). Moregenerally, the scaffold temperature may be between 5 and 15 degreesCelsius below TG for the polymer material. Whenever the scaffold iswithin the crimper head its temperature is at, or rose to the crimpingtemperature (e.g., 48+/−3° C.) for the crimping process described inFIGS. 1A-1B.

After the scaffold reaches the crimping temperature, the iris of thecrimper closes to reduce the scaffold diameter from 0.136 in (3.5 mm) toabout 0.11 in, or about a 20% diameter reduction. During this diameterreduction step (Stage I→Stage II) the balloon pressure is maintained atabout atmospheric temperature. The about 20% reduction in diameteroccurs over a period of about 5.2 seconds. As compared to subsequentdiameter reduction steps, this diameter reduction is performed moreslowly because strut angles are at their widest. It was found that aslow rate of diameter reduction can significantly improve yield, interms of more uniformity of compression in the scaffold structure; thatis, to enable the scaffold structure to compress more evenly, withoutirregular bending or twisting of strut and/or link structure. Furtherdetails on this aspect of the crimping process are described in U.S.application Ser. No. 12/861,719 filed Aug. 23, 2010.

Stage II:

The crimper jaws are held at the 0.11 in diameter, the balloon isinflated to a pressure of 150 psi, and the scaffold and balloon aremaintained in this configuration for a 30 second dwell period at thecrimping temperature. As explained earlier, the balloon is inflated to150 psi to help with stabilizing the scaffold structure and correctingfor any mis-alignment, or twisting of struts that might have occurredwhen the iris diameter was being reduced in size.

After the 30 second dwell period is complete, the balloon pressure isreturned to about atmospheric pressure and the crimper iris is movedfrom 0.11 in to 0.068 in or about a 38% diameter reduction. During thissecond diameter reduction or crimp step (Stage II→Stage III) the balloonpressure is maintained at about ambient temperature. This about 38%reduction in diameter occurs over a period of 1.0 second. The about 50%diameter reduction was found to achieve an acceptable balance betweenballoon-scaffold engagement while retaining an ability to re-align thescaffold in a Final Alignment step). If the scaffold is crimped tootightly before Final Alignment, then it becomes difficult to re-positionit between balloon markers. If crimped too loosely before FinalAlignment, then the scaffold can shift again after Final Alignment. Itwill be appreciated that this balance also should take intoconsideration the available spacing between struts for balloon material.

Stage III:

The crimper jaws are held at the 0.068 in diameter, the balloon is againinflated to a pressure of 150 psi, and the scaffold and balloonmaintained in this configuration for a 15 second dwell period at thecrimping temperature to correct or counter any twisting or misalignmentthat might have developed when the scaffold diameter was reduced byabout an additional 38%.

Final Alignment Step:

After the 15 second dwell period is complete, the scaffold and balloonare removed from the crimper to check the scaffold alignment on theballoon. This alignment involves a visual inspection and if necessarymanual adjustment of the scaffold to place it between the balloonmarkers. Alternatively, alignment may be performed by an automatedprocess, as explained in U.S. application Ser. No. 12/831,878 filed Jul.7, 2010.

As mentioned earlier, the scaffold's starting or pre-crimp diameter isabout equal to, or greater than the deployed diameter for the scaffold,which is between about 2.5 and 3.0 times its final crimped diameter. Theexpanded tube and pre-crimp scaffold diameter is 2.93 times thefinal-crimp size in the illustrated example. This difference indiameters between scaffold and balloon, coupled with the likelihood thatcrimper jaws will not apply a net-zero longitudinal force on thescaffold as the diameter is reduced, and/or that the scaffold will beslightly misaligned when it reaches the balloon surfaces, has lead to aneed for re-aligning, or verifying alignment of the scaffold on theballoon; that is, checking to see that the scaffold is located betweenballoon markers.

The additional, time-consuming alignment step that interrupts thecrimping process is typically not required for a metal stent, for tworeasons. First, the starting diameter for a metal stent is much closerto the final diameter, which means the balloon-stent interaction thatholds the stent in place happens relatively quickly. Second, for highercrimping rates used for metal stents, there is usually less ability forthe stent to shift longitudinally over the balloon surface. Metal stentscan be crimped at relatively high rates, whereas crimp rates for polymerscaffolds generally should be monitored and often times reduced (frommetal crimp rates) because a polymer scaffold's structural integrity inits crimped and deployed states is affected by the crimp rate. Whilemetals exhibit rate independent material behavior, polymers areviscoelastic and exhibit rate dependent material response. Polymerssubjected to higher strain or displacement rates will experience higherstresses and exhibit less ductility.

After Stage III the scaffold's diameter has been reduced to about ½ ofits starting diameter. It was observed that not until the scaffolddiameter was reduced to about 50% of its pre-crimp diameter size was thescaffold-balloon interaction sufficient to prevent longitudinal shiftingof the scaffold on the balloon when the scaffold was crimped downfurther. In the example of FIG. 1, therefore, the final-alignment stepis performed once the scaffold reaches about 50% of its pre-crimpdiameter. The Stage III diameter of 0.068 in places the scaffoldrelatively close to the deflated balloon to hold it in place, but nottoo tightly on the balloon so as to prevent re-alignment with respect tothe balloon alignment markers. When the scaffold and balloon are removedfrom the crimper to check alignment, the scaffold recoils from theballoon (e.g., from about 0.068 in to about 0.075 in).

Stage IV:

The scaffold and balloon are placed into the crimper. The jaws areclosed to a diameter of 0.07 in and the balloon inflated to a pressureof 70 psi (the pressure used for the intermediate pressure step in thisexample). Thereafter the scaffold is crimped to its final crimp diameterof 0.044 in or about a 33% reduction in diameter over a period of about2.6 seconds while balloon pressure is maintained at 70 psi. Before thefinal diameter reduction to 0.044 in commences, a dwell period of 10seconds at the 70 psi balloon pressure is performed to allow time forthe scaffold to return to the crimping temperature.

As illustrated in FIG. 1B, at the start of the Stage IV step the balloonpressure is set to 70 psi, and this setting is unchanged during thesubsequent Stage IV dwell, the subsequent diameter reduction from 0.07in to 0.044 in or about a 33% reduction (“intermediate pressure”), andthe Stage V dwell. The pressure is not adjusted to maintain 70 psi; assuch the balloon pressure is expected to change somewhat from 70 psiduring the intermediate pressure step.

Stage V:

After the scaffold has been reduced in diameter from 0.07 in to 0.044 inthe balloon pressure is maintained at 70 psi for a period of about 15seconds.

Following Stage V dwell period, the balloon pressure is returned toabout atmospheric pressure and the crimper jaws are held at the finalcrimp diameter for a 185 second dwell period. During this final dwellperiod the degree of recoil in the scaffold is reduced. Immediatelyfollowing the 185 second dwell the scaffold is removed and a retainingsheath is placed over the scaffold to reduce recoil.

Trials were conducted to estimate the likely pull-off or retention forcefor a 3.5 mm×18 mm PLLA scaffold crimped to a 0.044 in final crimpdiameter and crimped to a PEBAX balloon. The TABLE below shows resultsfrom these trials. The mean of the retention force for the scaffold for9 trials using the process of FIGS. 1A-1B was significantly higher thanthe mean of the retention force for the “control case”—i.e., the sameprocess as in FIGS. 1A-1B but without balloon pressure when the diameterwas reduced from 0.07 in to 0.044 in the final crimping stages. Thescaffolds used during these trials had substantially the same pattern asshown in FIG. 2. Statistics are shown for five Test cases.

Mean Standard Crimping retention deviation process force (lb) (lb) FIG.1A process Test case 1 1.32 0.33 Test case 2 1.63 0.11 Test case 3 1.430.16 Test case 4 1.56 0.17 Test case 5 1.55 0.12 FIG. 1A process withoutControl: 0.70 0.10 balloon pressure when diameter reduced from .07 to.044.

The results above were unexpected, since it was previously not believedthat a pressurization of the balloon during the final crimp by about ½the pressure of prior dwell periods would make much of a difference inthe retention force. The results demonstrate an about 30% to about 88%improvement in retention force over the “control” case.

As noted above, in a preferred embodiment a scaffold has the patterndescribed in U.S. application Ser. No. 12/447,758 (US 2010/0004735) toYang & Jow, et al. Other examples of scaffold patterns suitable for PLLAare found in US 2008/0275537. FIG. 2 of US 2008/0275537 shows a detailedview of a strut pattern depicted in US 2010/0004735. The intermediateportion includes rings 212 with linear ring struts 230 and curved hingeelements 232. The ring struts 230 are connected to each other by hingeelements 232. The hinge elements 232 are adapted to flex, which allowsthe rings 212 to move from a non-deformed configuration to a deformedconfiguration. Line B-B lies on a reference plane perpendicular to thecentral axis 224 depicted in US 2010/0004735. When the rings 212 are inthe non-deformed configuration, each ring strut 230 is oriented at anon-zero angle X relative to the reference plane. The non-zero angle Xis between 20 degrees and 30 degrees, and more narrowly at or about 25degrees. Also, the ring struts 230 are oriented at an interior angle Yrelative to each other prior to crimping. The interior angle Y isbetween 120 degrees and 130 degrees, and more narrowly at or about 125degrees. In combination with other factors such as radial expansion,having the interior angle be at least 120 degrees results in high hoopstrength when the scaffold is deployed. Having the interior angle beless than 180 degrees allows the scaffold to be crimped while minimizingdamage to the scaffold struts during crimping, and may also allow forexpansion of the scaffold to a deployed diameter that is greater thanits initial diameter prior to crimping. Link struts 234 connect therings 212. The link struts 234 are oriented parallel or substantiallyparallel to a bore axis of the scaffold. The ring struts 230, hingeelements 232, and link struts 234 define a plurality of W-shape closedcells 236. The boundary or perimeter of one W-shape closed cell 236 isdarkened in FIG. 2 for clarity. In FIG. 2, the W-shapes appear rotated90 degrees counterclockwise. Each of the W-shape closed cells 236 isimmediately surrounded by six other W-shape closed cells 236, meaningthat the perimeter of each W-shape closed cell 236 merges with a portionof the perimeter of six other W-shape closed cells 236. Each W-shapeclosed cell 236 abuts or touches six other W-shape closed cells 236.

Referring to FIG. 2, the perimeter of each W-shape closed cell 236includes eight of the ring struts 230, two of the link struts 234, andten of the hinge elements 232. Four of the eight ring struts form aproximal side of the cell perimeter and the other four ring struts forma distal side of the cell perimeter. The opposing ring struts on theproximal and distal sides are parallel or substantially parallel to eachother. Within each of the hinge elements 232 there is an intersectionpoint 238 toward which the ring struts 230 and link struts 234 converge.There is an intersection point 238 adjacent each end of the ring struts230 and link struts 234. Distances 240 between the intersection pointsadjacent the ends of rings struts 230 are the same or substantially thesame for each ring strut 230 of the strut pattern. The distances 242 arethe same or substantially the same for each link strut 234. The ringstruts 230 have widths 237 that are uniform in dimension along theindividual lengthwise axis 213 of the ring strut. The ring strut widths234 are between 0.15 mm and 0.18 mm, and more narrowly at or about 0.165mm. The link struts 234 have widths 239 that are also uniform indimension along the individual lengthwise axis 213 of the link strut.The link strut widths 239 are between 0.11 mm and 0.14 mm, and morenarrowly at or about 0.127 mm. The ring struts 230 and link struts 234have the same or substantially the same thickness in the radialdirection, which is between 0.10 mm and 0.18 mm, and more narrowly at orabout 0.152 mm.

As shown in FIG. 2, the interior space of each W-shape closed cell 236has an axial dimension 244 parallel to line A-A and a circumferentialdimension 246 parallel to line B-B. The axial dimension 244 is constantor substantially constant with respect to circumferential positionwithin each W-shape closed cell 236. That is, axial dimensions 244Aadjacent the top and bottom ends of the cells 236 are the same orsubstantially the same as axial dimensions 244B further away from theends. The axial and circumferential dimensions 244, 246 are the sameamong the W-shape closed cells 236.

It will be appreciated from FIG. 2 that the strut pattern for a scaffoldthat comprises linear ring struts 230 and linear link struts 234 formedfrom a radially expanded and axially extended polymer tube. The ringstruts 230 define a plurality of rings 212 capable of moving from anon-deformed configuration to a deformed configuration. Each ring has acenter point, and at least two of the center points define the scaffoldcentral axis. The link struts 234 are oriented parallel or substantiallyparallel to the scaffold central axis. The link struts 234 connect therings 212 together. The link struts 232 and the ring struts 230 definingW-shape closed cells 236. Each W-shaped cell 236 abuts other W-shapedcells. The ring struts 230 and hinge elements 232 on each ring 212define a series of crests and troughs that alternate with each other.Each crest on each ring 212 is connected by one of the link struts 234to another crest on an immediately adjacent ring, thereby forming anoffset “brick” arrangement of the W-shaped cells.

An iris-type crimper from Machine Solutions, Inc (MSI) was used to crimpthe scaffold according to FIGS. 1-2. The crimper includes three rollsarranged to position a clean sheet of non-stick material betweencrimping blades and the scaffold prior to crimping. The sheet is drawnfrom the backing sheet by a rotating mechanism (not shown) within thecrimper head. A second sheet is dispensed from a mid roll. Aftercrimping, the first and second (used) sheets are collected by a lowerroll. Within the iris two sheets are passed between two blades onopposite sides of the positioned scaffold and balloon, and a tension isapplied to gather up excess sheet material as the iris of the crimpingassembly is reduced in size via the converging blades. For the crimpingprocess of FIGS. 1A-1B the scaffold is crimped using this type ofcrimper. As explained in U.S. application Ser. No. 12/861,719 filed Aug.23, 2010 it was found that the polymer sheets disposed between thescaffold and crimper blades help to reduce indentations in scaffoldsurfaces since the polymer sheets effectively make the blade surfacesmore compliant.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects. Therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit and scope of this invention.

What is claimed is:
 1. A method for crimping a balloon-expanded scaffoldto a balloon, comprising the steps of: providing a tube comprising PLLAcharacterized by a glass transition temperature range having a lowerlimit of TG-low; radially expanding the tube to increase its radialstiffness; forming the scaffold from the radially-expanded tube; andusing a crimping mechanism, crimping the scaffold to the balloon whilethe scaffold has a crimping temperature of between 5 to 15 degrees belowTG-low including maintaining a pressure in the balloon as the scaffolddiameter is being reduced in size, the crimping including reducing thescaffold diameter from a starting diameter to an intermediate diameter,removing the scaffold from the crimping mechanism to adjust, ifnecessary, the position of the scaffold on the balloon before reducingthe scaffold to a final diameter, placing the scaffold back into thecrimping mechanism, and reducing the scaffold diameter to the finaldiameter, wherein a ratio of the starting diameter to the intermediatediameter is greater than a ratio of the intermediate diameter to thefinal diameter.
 2. The method of claim 1, wherein the balloon pressureis maintained when the scaffold diameter is being reduced to the finaldiameter.
 3. The method of claim 2, wherein the ratio of the startingdiameter to the intermediate diameter is about 50% and the ratio of theintermediate diameter to the final diameter is about 30%.
 4. The methodof claim 3, wherein the crimping temperature is about 48° C.
 5. Themethod of claim 1, wherein the crimped scaffold has a retention forcethat is greater than 1.0 lb, the scaffold has a length of no more than18 mm and the final diameter is at least 2.5 times less than thestarting diameter.
 6. A method for crimping a polymer scaffold to aballoon, comprising: using a crimping device, crimping the scaffold tothe balloon, including the steps of (a) reducing the scaffold diameterfrom a pre-crimp diameter to an intermediate diameter, (b) reducing thescaffold diameter from the intermediate diameter to a final diameter,and (c) maintaining a constant scaffold diameter for each of a pluralityof stress relaxation periods; wherein at least one stress relaxationperiod occurs after step (a) and before step (b); and wherein thescaffold diameter is not reduced by more than 40% between stressrelaxation periods.
 7. A method for crimping a balloon-expanded polymerscaffold to a balloon, comprising: using a crimping device, crimping thescaffold to the balloon, including the steps of reducing the scaffolddiameter by about 50%, after the about 50% diameter reduction, removingthe scaffold from the crimping device to inspect the scaffold, returningthe scaffold to the crimping device, and reducing the scaffold to afinal crimp diameter including maintaining an inflation pressure in theballoon while the scaffold diameter is being reduced to the finaldiameter.
 8. The method of claim 7, wherein the scaffold diameter priorto crimping is at least 2.5 times greater than the final diameter. 9.The method of claim 7, wherein the step of reducing the scaffolddiameter by about 50% includes at least one step of maintaining aconstant diameter of the scaffold within the crimping device after thediameter of the scaffold has been reduced from a pre-crimp diameter sizeto an intermediate diameter, less than the about 50% diameter reduction,wherein the at least one step of maintaining a constant diameter allowsfor stress relaxation in the polymer material prior to reducing thescaffold diameter by about 50%.
 10. The method of claim 7, wherein thescaffold has a pre-crimp diameter prior to reducing the scaffolddiameter by the about 50%, the crimping including a plurality of dwellperiods to allow for stress relaxation after the scaffold diameter hasbeen reduced, and the scaffold diameter is not reduced by more thanabout 40% between stress relaxation periods.
 11. The method of claim 6,wherein the scaffold diameter prior to crimping is at least 2.5 timesgreater than the final diameter.
 12. The method of claim 1, wherein thescaffold has a scaffold retention force of between about 1.3 and 1.6 lb.13. The method of claim 1, wherein the starting diameter is 3.5 mm andthe final diameter is 0.04 mm.
 14. The method of claim 13, wherein thecrimping from the 3.5 mm to 0.04 mm diameter includes a plurality ofdwell periods, each occurring after a diameter reduction in thescaffold, the dwell periods allowing for a stress relaxation in thepolymer material.
 15. The method of claim 14, wherein the dwell periodsrange from 1 sec to 185 sec.
 16. The method of claim 6, wherein thestress relaxation periods range from 1 sec to 185 sec.
 17. The method ofclaim 6, wherein the scaffold polymer is characterized by a glasstransition temperature (TG) and the scaffold has a temperature betweenTG and 15 degrees below TG during crimping.
 18. The method of claim 6,wherein the scaffold is cut from a polymer tube having a diameter,wherein after step (b) the scaffold is placed in a sheath, and whenremoved from the sheath the scaffold diameter is between 34% and 37% ofthe tube diameter.
 19. The method of claim 6, wherein the scaffold has aretention force of between about 1.3 and 1.6 lb.
 20. The method of claim6, wherein the scaffold is reduced in diameter by a factor of at least2.5.
 21. The method of claim 6, wherein the scaffold is crimped to aballoon such that a scaffold deployed diameter is at least 2.5 timesgreater than the final diameter.
 22. The method of claim 6, wherein thescaffold polymer is characterized by a glass transition temperature (TG)and the scaffold has a temperature between 5 and 15 degrees below TGduring crimping.